1. Field of the Invention
This invention relates to electrical current detection, and in particular to an indicating device that detects electrical current in the ground leg of a shore power cord connected between a land-based power supply and a docked marine vessel.
2. Description of the Prior Art
Large and even many small marine vessels have on-board electrical systems to power accessories including radios, navigational equipment, lights, pumps, refrigeration devices, battery chargers, and the like. When the vessel is at sea, the on-board electrical system is powered by a power source such as batteries, an on-board generator, an alternator on a drive engine, or a combination thereof. When docked, berthed, or otherwise in close proximity to land-based electrical power sources, vessel operators may augment the on-board electrical systems with power from a land-based source. The most common way of providing a ship-to-shore power connection is through the use of a shore power cord. The shore power cord provides a temporary but secure electrical connection between a land-based alternating current (AC) electrical source and a vessel's on-board electrical system.
In a typical marina, each dock or slip has a permanently mounted electrical receptacle specifically intended to provide power to a vessel. When power is needed on a docked boat, a shore power cord is used to connect the power source to the vessel. A conventional shore power cord includes a length of flexible, multi-conductor cable (e.g., hot, neutral and ground wires) having a watertight molded plug and connector at respective ends. The male plug is inserted into the land-based receptacle while the female connector is received by and locked to a charger or power inlet mounted to the vessel.
The hot wire supplies alternating current at a nominal voltage to power the on-board electrical accessories or recharge the batteries while the vessel is docked. The neutral wire is tied to earth, or shore, ground at a land-based power source (such as a circuit breaker) and provides a return path for the alternating current supplied by the hot wire. The ground wire is a redundant path to ground to help prevent electrified equipment and/or stray currents if any electrical accessories, conductors, or connections malfunction. As such, the grounding wire should not conduct electrical current during normal operation.
In a land-based electrical installation, the neutral wires and grounding wires are all connected to the same terminal strip (or bus bar) in the breaker box. In a marine electrical installation, the neutral wire cannot be connected to the vessel ground as doing so would provide an current path to ground through the surrounding water. Therefore, the grounding wire entering a vessel via a shore power cord is electrically coupled to the vessel's direct current (DC) grounded circuit. This circuit includes the negative (−) post of the vessel's DC electrical system. The negative post of the vessel's DC system is, by convention, tied to the vessel's bonding system. Therefore, in a vessel which is properly wired to utilize a shore power connection, the shore-based grounding circuit is electrically coupled to some or all of the underwater metal components of the vessel including propellers, shafts, rudders, outdrives, through-hull fittings, and the like. Though necessary to provide AC and DC leakage current a safe path to ground, the connection of the shore ground wire to the vessel's DC grounded circuit has the potential to create problems with galvanic and electrolytic corrosion.
Commercially available galvanic corrosion protection systems are used to protect underwater metal hardware against naturally occurring corrosion with blocks of alloy comprised primarily of the element zinc, or “zincs”. Zinc is a highly anodic metal, and typically far less noble (i.e., more galvanically active) than the metals, such as stainless steel, naval brass, and bronze, used for underwater fittings. Zincs are referred to as “sacrificial anodes” as zinc is more likely to give up electrons to seawater, and thus corrode faster, than the more noble alloys used in marine fittings.
The potential for greater galvanic corrosion increases when two vessels are hooked up to shore power and in close proximity to each other. If one of the vessels has any metal hardware dissimilar to the metal hardware on the neighboring vessel, a wet-cell battery effect is formed. The less noble metal hardware (e.g., an aluminum outdrive) of one vessel acts as a negative plate while the more noble hardware (e.g., a bronze rudder) of the second vessel acts as a positive plate. The surrounding seawater serves as an electrolyte and an electrical current is generated across the grounding wire of the shore power cord. The magnitude and speed of the corrosion is directly proportional to the difference of nobility between the opposing alloys.
The problem of electrolytic corrosion is similar to galvanic corrosion. In the case of electrolytic corrosion the degradation of the underwater metals is accelerated by a stray DC current emanating from one of the vessels. This may be caused by faulty wiring, a poorly grounded on-board appliance, etc. Electrolytic corrosion is often indiscernible from galvanic corrosion, and in fact the term galvanic corrosion is sometimes used to describe either type of corrosion.
Regardless of the technical classification of the corrosion, it is important to note that in most instances accelerated corrosion of underwater metal components on a vessel is directly related to the transfer of electrons (i.e., current) through the electrical connection, established through the grounding leg of the shore power cord, between adjacent vessels. Therefore, what is needed is a device that senses and alerts a vessel owner to the existence of a DC current on the grounding leg of the shore power cord. By doing so, excessive galvanic and/or electrolytic corrosion could be prevented by identifying a vessel operator of highly corrosive environments or conditions. Once these environments or conditions are recognized, preventative measures such as galvanic isolators, modified bonding systems, and additional sacrificial anodic protection can be put in place.
Ground fault circuit interrupters (GFCI), monitor the AC current between the hot and neutral wires of an electrical circuit. Under normal operation the current flowing into the circuit on the hot wire is equal to the current returning on the neutral wire. Any difference between the two currents exceeding a threshold value (e.g., 5 mA in a standard US GFCI-equipped outlet) is interpreted as a short circuit or other excessive leakage current-causing malfunction and power is removed from the hot leg of the circuit. However, GFCI devices do not monitor DC, i.e., galvanic, current, nor do they monitor the grounding leg of a circuit, where such current occurs. Moreover, GFCI devices are typically not used on shore power receptacles in the U.S. because the standard threshold trip currents (5 mA) are smaller than the currents which often ‘leak’ from on-board electrical systems, making them prone to tripping prematurely. Though functionally different, ground fault circuit breakers (GFCB) and residual current devices (RCD) do not protect against galvanic corrosion for the same reasons as with a GFCI. Therefore, a need exists for a device to monitor the grounding leg of a shore power cord and provide an indication as to the existence of corrosion-causing currents therein.
Galvanic isolators, are devices installed in series with the grounding wire of the shore power cord to block low voltage DC galvanic current flow while still permitting the passage of alternating current normally associated with the grounding wire. In doing so, a galvanic isolator allows the grounding leg of the shore power system to operate as a redundant path for return (ground) currents while preventing the conduction of DC currents normally associated with galvanic corrosion. However, galvanic isolators are often installed only after an appreciable level of corrosion has already occurred. Therefore, a need exists for a device to monitor and provide an indication as to the existence of electrical current potentially indicative of accelerated corrosion.